(a) Field of the Invention
The present invention relates to a communication method. More particularly, the present invention relates to a communication method and a beam forming apparatus in a mobile communication system having a multi-cell.
(b) Description of the Related Art
Mobile satellite communication systems provide a broadcasting service in downtown and suburban areas through an integrated satellite and terrestrial network using a terrestrial auxiliary device such as a repeater, a complementary terrestrial component (CGC), and an ancillary terrestrial component (ATC). In this case, the mobile satellite communication system provides a broadcasting service through a satellite in a rural or suburban area in which a line of sight (LoS) is secured and provides a broadcasting service through a terrestrial auxiliary device in a downtown or indoor environment in which a satellite signal is not secured.
In the mobile satellite communication system, a radio wave that a satellite radiates to an earth surface area is referred to as a beam, and the earth surface area to which the beam is radiated is referred to as a cell. The mobile satellite communication system includes a single beam satellite communication system in which a satellite uses one beam, and a multibeam satellite communication system in which a satellite uses two or more different beams. The multibeam satellite communication system divides a service area into several cells by multiple beams like a terrestrial cellular mobile communication system based on a base station, and a mobile terminal receives a service through a frequency corresponding to a beam radiated to where the mobile terminal is located.
Nowadays, as requests for a high definition multimedia service increase, the mobile satellite communication system should provide a wideband service. However, recently, the bandwidth that is allocated for mobile satellite communication services has become very limited. For example, a satellite IMT-2000 band that is allocated to ITU-R (radio-communication sector) is the 30 MHz band for each of an uplink and a downlink. Specifically, 1980-2010 MHz is allocated to the uplink, and 2170-2200 MHz is allocated to the downlink.
In order to provide a wideband service, because a radio interface having a bandwidth of at least 10 MHz is considered, the frequency that is allocated to embody a frequency reuse ratio of 3 or 7 is very limited. For example, it is difficult to embody a frequency reuse ratio of 7, and there is a problem that a frequency of all bands should be allocated to an operator having a frequency reuse ratio of 3. Therefore, in order to provide a wideband service, it is essential to embody a mobile satellite communication system having a frequency reuse ratio of 1. Unlike a code division multiple access (CDMA)-based mobile communication system, in an orthogonal frequency division multiplexing (OFDM)-based satellite communication system, due to an interference problem between adjacent cells, a frequency reuse of ratio 1 is not generally, satisfied and thus in a cellular communication environment, the OFDM-based satellite communication system has characteristics that are less appropriate than those of the CDMA-based mobile communication system.
In order to solve this, by enabling a terrestrial network to satisfy a frequency reuse ratio of 1 through several fractional frequency reuses, the terrestrial network is appropriate for a cellular environment. Specifically, by dividing one cell into several areas and by enabling each area to use a portion of subcarriers, even in a situation of a frequency reuse ratio of 1, a method of reducing interference between adjacent cells was used.
However, in this way, a method of use in a terrestrial network can be applied under an assumption that much difference exists in a path loss value in a base station vicinity area and a cell boundary area and that a signal may be separated and transmitted to each divided area of a cell, as each sector has an antenna. Therefore, a fractional frequency reuse method that is applied to a terrestrial network cannot be applied to a satellite network in which a signal cannot be separated on an area basis within one beam.
As technology about a fractional frequency reuse method in a satellite network, there is an “Apparatus and method of mobile satellite communication comprising complementary terrestrial component applied fractional frequency reuse technique” that is disclosed in Korean Unexamined Patent Application Publication No. 2010-0058907. In this technology, by enabling a mobile terminal and a satellite to communicate using different subcarrier groups on a beam basis, a fractional frequency of a subcarrier group that other beams do not use may be reused, and each beam performs communication between a mobile terminal and a terrestrial auxiliary device using a subcarrier group that is not used when a mobile terminal and a satellite communicate.
However, as such technology uses a partial frequency in a beam boundary area, there is a drawback that spectral efficiency is deteriorated and an interference problem from another beam occurs, and thus a limitation exists in terms of entire system capacity.